Gamma-prime (γ′) precipitation-strengthened nickel-base superalloys such as IN738LC, MAR-M 247 and CM247LC are known, which are designed to be resistant to creep deformation for high-temperature applications up to about 950° C. to 1050° C.
Such superalloys, however, are difficult to weld due to their sensitivity to microcracking. Microcracking of nickel-base superalloys during welding is generally attributable to solidification cracking in the mushy zone, liquation of precipitates or low-melting point eutectics in the heat-affected zone (HAZ), ductility dip cracking or strain-age cracking in subsequent heat treatment and embrittlement of the HAZ by segregation of detrimental impurities at interfaces.
B Geddes et al., “Superalloys, Alloying and Performance”, ASM International, 2010, pages 71 to 72, the contents of which are incorporated herein by reference, describe a weldability limit for superalloys where [2 times Al concentration in wt %+Ti concentration in wt %] is approximately 6.0. Superalloys above this limit are considered as difficult to weld materials. Solidification and grain boundary liquation cracking occurs during the welding process of such materials, whereas post-weld heat treatments often lead to strain age cracking in gamma-prime Ni3(Al, Ti) precipitate strengthened alloys.
Generally speaking, strain-age cracking can be due to volume changes from gamma-prime precipitation, which occur during repeated thermal cycles and give rise to significant residual stresses. Solidification cracking can be caused by the presence of residual liquid during thermal cycles. Constitutional liquation cracking can be caused by rapid heating of relatively low melting point phases, such as carbides, borides and gamma-prime, at interfaces and interdendritic regions. Interface embrittlement from sulphur as a result of low melting point Ni3S2, which can “wet” interfaces and grain boundaries and give rise to brittle intergranular fracture for a range of temperatures, can also cause cracking in nickel-base superalloys. These problems have restricted the development of processes for fast and convenient fabrication of relatively complex metal articles, components and structures, for example using additive manufacturing processes such as three-dimensional (3D) printing, because the repeated thermal cycles encountered in such processes create stresses that will cause cracks to appear in the crack-prone metal. In addition, the conventional nickel-base superalloys produce metal products that are likely to show low levels of ductility from tensile loading and creep deformation, notably over temperatures between about 650° C. and 1050° C. These problems are typically caused by damage from inelastic strain localised to interfaces, for example grain boundaries, and the gamma-phase precipitates.
European Patent Application No. 2949768, the contents of which are incorporated herein by reference, describes a nickel-base superalloy in which the ratios of the alloying elements C, B, Hf, Zr and Si are as follows:
C/B10-32C/Hf>2C/Zr>8C/Si >1,
for example in an alloy consisting of the following components (wt %):
Cr7.7-8.3Co5.0-5.25Mo2.0-2.1W7.8-8.3Ta5.8-6.1Al4.7-5.1Ti1.1-1.4C0.08-0.16, preferably 0.09-0.16, most preferably 0.13-0.15B0.005-0.008, preferably mostpreferably 0.005-0.008Hf0-0.04, preferably 0-0.01, most preferably 0Zr0-0.01, most preferably 0Si0-0.08, preferably 0-0.03,most preferably 0,
the balance nickel and unavoidable impurities.
The alloy is in the form of a powder wherein the powder size distribution is between 10 and 100 μm and a spherical morphology.
Such a superalloy powder is said to be suitable for additive manufacturing of metallic articles, for example by selective laser melting (SLM) or electron beam melting (EBM). In particular, the addition of carbon as a grain-boundary strengthener in preference to B, Hf and Zr as possible alternative grain-boundary strengtheners is said to improve crack-free processing by additive manufacturing methods. The superalloy is a modified version of the nickel-base superalloy “L1” disclosed in European Patent Application No. 1359231 (“A1” in the corresponding U.S. Pat. No. 6,740,292), which corresponds to it in the amounts of the components Cr, Co, Mo, W, Ta, Al, Ti, C and B and differs from it in that “L1” (“A1”) has 0.02 wt % C, 0.005 wt % B, higher amounts of Hf and Si and no Zr.
The above proposal may go some way towards providing additive manufacturing processes using an alternative to “L1”. However, it does not address the desirability of improving the cracking resistance of other nickel-base superalloys during high temperature manufacture, processing or use.
The present invention is based on our surprising finding that by controlled adjustment of a specific combination of components in the nickel-base superalloy CM247LC, a novel alloy is obtained which combines the strength advantages conferred by the presence of large volumes of gamma-prime precipitation with a reduced propensity for cracking during high temperature manufacture, processing or use. As a result, the novel modified form of the superalloy CM247LC enables this alloy to be fabricated into metallic articles, components and structures using a variety of high temperature forming processes, including additive manufacturing methods and powder-based manufacturing methods, including methods where thermal cycling is expected.
CM247LC is described in Harris et al., “MAR-M 247 Derivations—CM247LC DS Alloy, CMSX Single Crystal Alloys, Properties and Performance”, 5th Intl. Symposium, October 1984, pages 221-230, the contents of which are incorporated herein by reference. Its nominal composition is as follows:
Cr8.1Co9.2Mo0.5W9.5Ta3.2Al5.6Ti0.7Hf1.4C0.07B0.015Zr0.015
the balance nickel and unavoidable impurities. Upper limits of manganese (Mn), sulphur (S) and niobium (Nb) in CM247LC are generally Mn 0.05 wt % max, S 0.003 wt % max, and Nb 0.10 wt % max.